The present invention relates to a method for decontaminating radioactively contaminated surfaces of metallic materials with the use of nitric acid and hydrofluoric acid.
In a summarizing report entitled "Kontamination und Dekontamination von Oberflachen," in translation, "Contamination and Decontamination of Surfaces", in Kernenergie, in translation, Nuclear Energy, Vol. 5, 1962, pages 585-600, H. F. Moldenhawer describes a number of different decontamination methods and a number of decontamination agents. The large majority of the listed decontamination agents are used in the form of aqueous solutions. Moldenhawer points out that strong inorganic acids belong to the most effective, but also the most aggressive, decontamination agents. Nitric acid or HNO.sub.3 containing solutions are listed in first place. Moldenhawer states, however, that it is error to draw the conclusion that pure nitric acid would be the best decontamination agent. For example, Moldenhawer mentions, among others, a solution of 3% HF+20% HNO.sub.3, referred to as a 3-20 reagent, as a decontamination agent for stainless steel. For other surfaces, completely different solutions are used.
German Patent No. 23 33 516 discloses a vapor phase method for decontaminating radioactively contaminated metallic surfaces wherein the surfaces are brought into contact with vapor containing substances which reduce surface tension. This method employs a decontamination agent in the form of a vapor phase mixture of water vapor, nitric acid vapors and water vapor volatile, perfluorated, aliphatic acids and/or salts of these acids, and the like. The vapor phase mixture has a nitric acid concentration below that of the azeotropic mixture and a perfluoro carboxylic acid content in the vapor phase corresponding to that which reduces surface tension in the condensate at 293.16.degree. K. to an order of magnitude of 250 .mu.N/cm. The concentration of nitric acid in the vapor phase preferably corresponds to a concentration in the range between 2 and 6 mol/l HNO.sub.3, measured in the condensate, and the content of perfluoro carboxylic acid and/or its salts in the vapor phase corresponds to a concentration in the range between 100 and 300 ppm. In this method, a mother solution is heated to the boiling point to form the decontamination agent, the boiling temperature is maintained until the decontamination process is completed, and care is taken to avoid contact of the mother solution with the surfaces or articles.
The known decontamination methods, as they are practiced in processing systems, pipelines, tanks, boilers, heat exchangers, and the like, sometimes produce only a low decontamination effect. The amounts of waste in the form of spent decontamination agents are sometimes relatively large, particularly for decontamination by way of immersion in a bath. Moreover, the decontamination agent wastes are sometimes poorly compatible with customary fixing methods for radioactive liquid wastes.